This invention relates, in general, to circuits requiring voltages exceeding a power supply voltage, and more particularly, to charge pump voltage circuits.
A charge pump circuit is a circuit that generates a voltage greater than it receives. In general, a charge pump circuit works through the alternate steps of charging a charge capacitor and then placing the charge capacitor in series with a voltage source or other voltage element (such as a second charged capacitor) to create an increased voltage.
The charge capacitor is coupled from a charge configuration to a series configuration by switches. The switches are controlled by a control logic circuit. Higher voltages may be generated by charging more than one capacitor and placing them in series to generate a desired voltage.
In general, a storage capacitor is used in conjunction with the charge capacitor for providing a continuous voltage to a circuit. For example, a storage capacitor is coupled to a circuit for providing a voltage greater than a power supply voltage. A charged storage capacitor cannot sustain a voltage to the circuit indefinitely, it must be recharged to maintain the voltage. A charge capacitor in a series configuration is used for charging the storage capacitor.
For example, in a first cycle, a charge capacitor is charged up to a power supply voltage. A storage capacitor maintains a voltage to a circuit during the first cycle. The voltage provided by the storage capacitor decreases as it discharges due to the circuit loading. In a second cycle, the charge capacitor is coupled in a series configuration with the power supply voltage (or other voltage source) to generate an increased voltage. The charge capacitor is then coupled to the storage capacitor for charging the storage capacitor. The charge capacitor is decoupled from the storage capacitor after providing charge to increase the voltage of the storage capacitor. The first and second cycles are continuously repeated to maintain the voltage on the storage capacitor above a minimum value. The size of the charge and storage capacitors and the frequency in which the first and second cycle is repeated is dependent on the circuit loading.
An example of a device requiring voltages greater than a power supply voltage normally provided for semiconductor circuits is a Liquid Crystal Display (LCD). A power supply voltage Vdd is provided to power integrated circuits that operate in conjunction with the LCD. Four logic levels are required in the operation of a typical LCD (Ground, Vdd, 2*Vdd, and 3*Vdd). A charge pump circuit that triples the power supply voltage is used to generate the 2*Vdd and 3*Vdd voltages. If the charge pump circuit is integrated with an integrated circuit (operating at the power supply voltage Vdd) there is a possibility that the voltage generated by the charge pump circuit could exceed device specifications of the integrated circuit. The stress on the devices in the charge pump due to the elevated operating voltages could degrade device performance or worse a catastrophic failure could occur.
It would be of great benefit if a circuit and method could be provided for operating a charge pump circuit such that the voltage on the devices within the charge pump circuit is reduced.